Natural killer (NK) cells are essential immune effector cells due to their intrinsic capacity to detect and destroy virus-infected and malignant target cells. In control of this essential function is an array of highly polymorphic cell surface receptors that regulate NK cell effector functionality. Human genetics studies have shown that certain combined gene sets for killer immunoglobulin-related receptor (KIR) NK-cell receptors and their HLA class I ligands affect clinical outcomes in autoimmunity, infection, transplantation and pregnancy syndromes. However, the molecular and cellular bases of these effects are not understood. Our prior mouse genetics studies have also found that significant genetic interaction between gene sets for NK receptors and MHC class I ligands controls NK cell features, and possibly their capacity to mediate critical antiviral functions and protection from immune- or virus-mediated tissue damage and disease. A MHC-linked quantitative trait locus (QTL), Cmv5, that specifically regulates NK cells, secondary lymphoid organ (SLO) structures, and tissue necrosis in spleen after viral infection was also recently discovered. However, it is not known how Cmv5 traits are regulated, how distinct alleles affect histopathological outcomes, or if genetic control manifests through hematopoietic cells, non-hematopoietic cells, or both. We hypothesize that Cmv5 regulation of NK cells and lymphoid architecture during acute virus infection can profoundly impact the tempo/extent of viral clearance and tissue inflammation. Likewise, Cmv5 may affect priming of adaptive immunity, memory formation and ultimate disease outcomes. The broad long-term objectives of this research proposal, therefore, center on identifying and fully characterizing MHC I Dk, NKC and Cmv5 genetic modifiers to elucidate how genetic variation controls NK cells, SLO structures and tissue necrosis in infected tissues following acute viral infection. To achieve the objectives, Aim 1 investigates molecular and cellular bases of Cmv5s suppression of NK expansion and SLO structure protection during inflammation and pathogen infection. Classical genetics and genetic screens will be applied to precisely map a Cmv5 critical interval. Exome sequence variant analyses, and chromosome editing strategies will be applied to further minimize the interval by constructing Cmv5 interval/candidate sequence deletions and testing for loss of suppression mutations in experimental cross mice. Aim 2 explores the NKC effect on Cmv5 regulation of NK expansion and SLO protection in MCMV- infected spleen. We hypothesize that strain-specific regulation of Cmv5 effects is controlled at the level of NK cell effector functionality, and reactivity to viral infection. NKC variation, resulting in NK receptor diversity, will be carefully assessed for its effect on Cmv5 regulation of NK cells, SLO structural integrity and tissue necrosis during viral infection in congenic, transplanted and NK receptor gene-edited mouse strains. Our studies should ascertain how NKC and MHC genetic interaction specifically shapes NK cells, additional critical immune cells, and inflammatory responses, which may help to model combined KIR/HLA associations with clinical response.